This invention relates generally to a method to make light-weight metal-matrix composite components through squeeze casting or semi-solid metal (SSM) forming, and more particularly to making such components from a reinforced metal matrix composite where the reinforcing phase or phases are generated in-situ during such casting or forming operations.
Casting has become the dominant form of metal-forming operations for the manufacture of repeatable (i.e., high-volume) components (particularly those that employ lightweight metal alloys such as aluminum or magnesium), and includes numerous variants, such as die casting, permanent mold casting, sand casting, plaster casting, investment casting or the like. Nevertheless, it is known that the mechanical properties of cast components are often inferior to their wrought counterparts, due in no small part to porosity and related defects that are inherent in (or at least hard to avoid) known casting processes. Unfortunately, high-volume production and shape complexity considerations may render wrought options cost-prohibitive, if not outright impossible.
SSM forming techniques have helped to bridge the gap by providing metallic alloys that deliver wrought properties with a forming process capable of the large-scale production of complex shapes. In particular, the slurried (i.e., thixotropic) microstructure of these SSM techniques makes it easy to perform the semi-solid shaping by casting, forging or other known forming processes. In a conventional SSM forming process, a cast billet is (1) heated to a temperature above its recrystallization temperature yet below its solidus temperature; (2) extruded into a generally columnar form; (3) cut into shorter segments; (4) heated into a semi-solid state; and (5) squeezed into a cavity that is formed in a die set to form a part. Despite advantages, porosity, outer skin microstructure and related incomplete part formation issues persist in conventional SSM, especially in articles formed with complex geometries with thin or otherwise small features. Moreover, the billets used in this and related thixotropic processes are a highly specialized (and therefore expensive) way to achieve the desirable non-dendritic (i.e., globular) microstructure.
In a related way, squeeze casting has been investigated as a way to prepare components from lightweight alloys. The process is also referred to by other names, such as liquid metal forging, liquid die forging, semi-solid casting and forming, extrusion casting, pressurized solidification and pressurized crystallization. A conventional squeeze casting process is defined by the following steps: (1) pre-quantifying an amount of melt to be poured into a preheated die cavity; (2) ramping down a punch close to the die cavity; (3) pressurizing the molten metal and holding it there for a short period (for example, a few seconds) until the punch is withdrawn; and (4) ejection of the part from the die cavity. Thus, in one form, squeeze casting (and related liquid forging approaches) is simpler than SSM forming in that it uses a pre-determined volume of molten metal that is poured into a die cavity and squeezed under pressure during solidification, thereby forming the alloy parts in a single operation. Moreover, squeeze casting makes it possible to use wrought aluminum (or magnesium) alloy in a liquid state to form complex parts with intricate features. High direct melt pressure helps eliminate hot tearing and creates products with superior mechanical properties and low porosity. As such, squeeze casting is seen as a hybrid of conventional casting and forging techniques to achieve the strength and confidence level of forging with the high-volume economics and shape capabilities of castings.
It is known that increased structural or mechanical properties (such as elastic modulus, strength, fatigue resistance, creep resistance or the like) of components may be achieved through the introduction of reinforcing phases into the bulk alloy. As such, the class of materials known as composites has been created to help satisfy increasingly these and other demanding engineering requirements. One of the difficulties associated with creating such engineered composites is the cost associated with introducing disparate materials in such a way that they achieve the desired structural benefits in the final product. Because the introduction of a discreet reinforcing phase into a bulk alloy is complex (and therefore prohibitively expensive), it is incompatible for high-volume component production techniques for engine components through one or more of the traditional forms of metal casting mentioned above.
Significantly, the present inventors have discovered that traditional SSM or squeeze casting techniques have not been able to fully exploit all of the mechanical or structural properties that the use of such materials would otherwise offer. Specifically, the present inventors have determined that there remains a need to develop low-cost, durable engine components through a cost-effective, high-volume manufacturing approach that uses SSM, squeeze casting or related fabrication techniques to better exploit the high specific properties made possible by lightweight metal matrix composites.